1. Field of the Invention
The invention relates to encapsulating electronic components or circuits, and in particular to encapsulating integrated circuits.
2. Description of the Prior Art
Several techniques for encapsulation of integrated circuits are known in the art. In one well known process a leadframe is fabricated from a thin copper sheet, a circuit chip or die is attached, and the leadframe and chip are encapsulated in a molded package. In the molding step, a hollow mold pattern is clamped around the chip from the upper and lower surface of the leadframe, and the mold is filled with molding compound, or encapsulation, to create the package body. Unfortunately, during the molding step, the encapsulating material oozes out of the mold joint into the region between the leads. This extra encapsulant, or "flash," is undesirable because it can later flake off and jam equipment used to insert the completed integrated circuit package into printed circuit boards. The flash also can prevent the completed package from fitting into a socket or, in the case of a surface mount package, can interfere with bonding the encapsulated package to a circuit board.
One solution to the flash problem has been to punch rectangular holes in the leadframe to create a barrier strip or "dambar" in the copper leadframe well outside the package. During encapsulation, the extra encapsulant fills the holes between the package body and the dambar and goes no further. After encapsulation, the extruded flash is punched away with a "deflash" punch, and in a subsequent operation, the copper dambar is removed, usually by punching. The resultant package, however, typically still has residual portions of flash extending as much as 20 mils.
Because the encapsulating material is abrasive, the deflash punch must be replaced frequently, typically every 50,000 operations. In practice, this can mean halting the assembly process as often as three times per day to replace a worn-out punch head.
A further disadvantage of the deflash punch is that because production tolerances vary, the deflash punch head is not allowed to come closer than about 5 mils to the edge of the encapsulated package. Less separation, under worst case conditions, can result in the deflash punch head striking the encapsulated package. Even with a 5 mil separation distance, however, deflashing can fracture the package because the flash is integral to the package and adheres well to the leadframe material. Package fracture during deflash punching decreases yield at the worst possible time, when the product is nearly completed.
The prior art described above is illustrated in FIG. 1. FIG. 1A shows an integrated circuit leadframe 4 prior to encapsulation. The outline of the packaged body 12 (not yet molded) is shown in phantom. Leadframe 4 has a number of leads 6 which are connected to an integrated circuit which will be disposed in the center. Leads 6 extend outward to become pins 7 for the ultimately packaged product. Leadframe 4 has holes 8 between adjacent leads 6, which are defined by a dambar strip 10, a distance 14 from the package edge. Typically distance 14 is 15-20 mils, while dambar 10 is about 15-20 mils wide.
During encapsulation, body 12 is formed by molding an encapsulant about the leadframe 4, totally covering the integrated circuit chip. FIG. 1B is an expanded view of a portion of FIG. 1A after molding the package 12. As shown in FIG. 1B, unwanted flash 16, much thinner than the package body 12, fills holes between the sides of package 12 and the dambar 10. On one hand, if it is desired to have minimal flash extending from the finished package, dambar 10 should be located a very small distance from edge of package 12. On the other hand, however, too small a distance 14 requires the deflash punch to come very close to the package while removing the flash, thereby increasing the likelihood of fracturing the package.
As shown in FIG. 1C, during the next manufacturing step the flash 16 is punched away with a deflash punch, leaving only a small portion 16 protruding from the package. In a subsequent step, the dambar 10 is punched away to electrically isolate the pins leaving a completed package.